The present invention relates generally to metal cutting tools and more particularly to a tool for countersinking a hole in a workpiece composed of metal, composite materials or the like.
The present invention is especially concerned with a tool for providing the countersunk part of the hole with at least two vertically aligned, outwardly flaring, axially adjoining portions with each portion extending at a different angle relative to the axis of the hole. When the countersunk part of the hole is so formed, it will accommodate the head of a fastener (such as a rivet) having a shank portion from which extends a terminal or head portion flaring outwardly from the shank portion at an angle. In the countersunk part of the hole, the upper outwardly flaring portion engages the head portion of the fastener, and the lower outwardly flaring portion provides a gradual transition between the cylindrical part of the hole and the upper outwardly flaring portion. The provision of a lower outwardly flaring portion eliminates the presence of a sharp corner between the cylindrical part of the hole and the upper outwardly flaring portion. Such a sharp corner could prevent the head of the fastener from being positively seated in the countersunk part.
Conventional countersinking tools have included a rotating shaft on which was brazed a counter-sinking element having a profile conforming to the profile desired in the countersunk part of the hole. However, this type of countersinking element had a relatively large initial expense. Moreover, when it wore down, it had to be removed from its rotating shaft, reground, and then reset on the rotating shaft. The regrinding operation for this countersinking element was relatively expensive. Regrinding was necessary, however, because the large initial expense of the element made it too expensive to be thrown away after it was worn down.
There was another type of element used for the countersinking operation which was initially less expensive than the non-disposable element described above and which could be thrown away after it wore down (e.g., after countersinking 100 holes). However, the initial cost of this disposable element, although substantially less than the cost of the non-disposable element, was still relatively high. More particularly, the disposable element initially cost only about half as much as the non-disposable element, but the initial cost of disposable element was still well over twice the cost of regrinding the non-disposable element. Thus, neither of the prior art countersinking elements was particularly advantageous over the other, from a cost standpoint.